The steps to change the master password and investor password on mobile phone are the same.
Steps: After successfully logging into the MT4/MT5 apps, click on "Setting" on lower right corner, and click your "real account" on the top of your screen.
Then click right side "···”
Click "Change Password" when the page pop-up
Choose "Change Master Password" or "Change Investor Password"
Input "Current master password" and type new master/investor password, click "Change" on the top right corner afterwards.